Gian Andreossi played his entire career with EHC St. Moritz, from 1921-29, winning Swiss Championships with them in 1922, 1923, and 1928. He competed for Switzerland four times at the European Championships, winning a gold medal in1926 and bronze medals in 1922 and 1924. He played forward for Switzerland at the 1928 Olympics, appearing in five games. His brother, Mezzi Andreossi, also played ice hockey for Switzerland at the 1928 Winter Olympics. A third brother, Zacchi, never played in the Olympics, but tended goal for Switzerland at the 1922 European Championships. The three brothers were on that Swiss team, making it the first trio of brothers to appear on any national team.